Chemical Company salesman manipulate half-life determinations to their own ends. Chemicals don’t simply evaporate at their half life time. It is what they break down into that is also critically important. DDT has a half life as little as 22 days.
So the chemical companies can say that in as little as 22 days, half the DDT is gone. And in 8 times 22 days or, just over six months, it’s all gone.
But it is not! DDT breaks down in “as little as 22 days” _into DDE_. The DDT is gone but DDE remains. And they know it. And DDE is thought to be more toxic and longer-lived than original DDT.
In particular it is the DDE that caused the bird egg shell thinning.
Source: https://en.m.wikipedia.org/wiki/DDT#Environmental_impact
The biological thinning mechanism is not entirely understood, but strong evidence indictates that p,p’-DDE inhibits calcium ATPase in the membrane of the shell gland and reduces the transport of calcium carbonate from blood into the eggshell gland. This results in a dose-dependent thickness reduction.[1][61][62][63]
Also
Source: https://en.m.wikipedia.org/wiki/Dichlorodiphenyldichloroethylene
DDE is particularly dangerous because it is fat-soluble like other organochlorines, thus it is rarely excreted from the body and concentrations tend to increase throughout life. The major exception is the excretion of DDE in breast milk, which delivers a substantial portion of the mother’s DDE burden to the young animal or child.[3] Along with accumulation over an organism’s life, this stability leads to bioaccumulation in the environment which amplifies DDE’s negative effects.